Abigail and Shaun Bengson's songs are fiercely engrossing, Kate E. Ryan's book is densely grounded, director Anne Kauffman's production is inventively solid, and everything is in flux in "Hundred Days."

That's not just because the transience of life and love is the theme of the dynamic indie-folk rock musical that opened Saturday at Z Space. The show is in transition. As lead producer, Z Space's Lisa Steindler told the opening-night audience, the world premiere - co-presented with Piece by Piece Productions and Encore Theatre - is still something of a work in progress, with changes to be made throughout the run.

That may be a good reason to see it more than once, given the magnetic vitality of the music and the extraordinary talent onstage. The story still needs work, particularly the ending, and the songs performed will vary from the list in the program, but Abigail Bengson's gripping, chilling, exhilaratingly wild and heartrending "Three Legged Dog" solo alone - the plaint of a woman facing life without her husband - is a must-see. And it's not the only one.

"Days" is the story of Sarah and Will, newly met, newly married and facing his sudden diagnosis of terminal cancer (delivered with hair-raising playfulness in "Creature"). The husband-and-wife Bengsons embody the couple with full-throated, eclectic vocals and engaging spontaneity, shadowed by Amy Lizardo and Reggie D. White in song and Joe Goode's subtle choreography.

Everyone but the Bengsons serves as the potent chorus, and most get at least one solo in a dense, varied, almost sung-through score that covers an indie-rock map from folk, blues, Irish and a touch of gospel to art song. Kate Kilbane (of the Kilbanes), El Beh, Joshua Pollock and Geneva Harrison make up the impressive band.

The way "Days" sags a bit early in the second act exposes a problem with its central conceit - a static selfishness in the couple's attempt to cram a full life into the time Will has left. But that also sets up both Bengsons' and Lampert's richest, most compelling and virtuosic arias. It could still be better, but most of "Days" is a fresh, invigoratingly tuneful wrestle with the existential conundrum that life is the process of dying.

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